[WIP] Added sonata_type_model_autocomplete form type

``sonata_type_model_autocomplete`` is additional model form type for ``sonata_type_model`` and ``sonata_type_model_list`` for handling many-to-one relations.

I've created it because ``sonata_type_model`` can't be used for too many items and ``sonata_type_model_list`` is not ideal for some use cases.

Usage is very simple:

```php
    // e-mail is a property from related User entity, on which we will filter the data
    $formMapper->add('user', 'sonata_type_model_autocomplete', array('property'=>'eMail'));

    // or if you need change the query, you can specify callback option
    $formMapper->add('user', 'sonata_type_model_autocomplete', 
        array(
            'property'=>'eMail', 
            'callback'=>function($queryBuilder, $alias, $property, $value){
                        $queryBuilder->andWhere($alias.'.enabled=1');
                    }));

    // there are additional options, like ``url``, ``route_name``, ``placeholder``, ``minimum_input_length``, ``items_per_page`` ... 
```
| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1172, #1758, partially #633
| License       | MIT
| Doc PR        | -

TODO: 
- documementation
- basic tests